1. Field of Invention
This invention relates to an antiskid brake control system in an automotive vehicle. More particularly, the invention relates to an antiskid brake control system provided with flow passage selector valves and pressure compensated flow control valves, in which the selector valves change over to a slow/fast depressurization mode to reduce brake hydraulic pressure and a slow/fast pressurization mode to raise brake hydraulic pressure at the time of antiskid braking control.
In general, antiskid brake control is adapted so that when it is detected during braking that the wheel of a vehicle is about to skid, the braking force acting upon the wheel is reduced to prevent skidding, after which the braking force is again enlarged. Such control stabilizes vehicle steering and makes the braking distance as short as possible.
2. Description of the Prior Art
One example of this antiskid brake control system for performing such a brake control operation is as shown in FIG. 5.
As illustrated in FIG. 5, this antiskid brake control system has a brake circuit formed into a diagonal piping configuration comprising a first brake system A and a second brake system B. The first brake system A is adapted so that the hydraulic pressure of a hydraulic-pressure source 1 including a pump 1a and an accumulator 1b is supplied to a wheel cylinder 3 for the right front wheel and a wheel cylinder 4 for the left rear wheel via a first control valve 2. The second brake system B is adapted so that the hydraulic pressure of the hydraulic-pressure source 1 is supplied to a wheel cylinder 6 for the left front wheel and a wheel cylinder 7 for the right rear wheel via a second control valve 5. Since the first and second brake systems A and B have the same construction, the first brake system A will be described and a description of the second brake system B will be omitted.
In the first brake system A, the first control valve 2 is constituted by a three-port, three-position valve. An output port 2a of the control valve 2 and the wheel cylinder 3 for the right front wheel are communicated via a first supply passageway 8, and the output port 2a of the first control valve 2 and the wheel cylinder 4 for the left rear wheel are communicated via a second supply passageway 9.
The first and second supply passageways 8, 9 are respectively provided with a first flow passage selector valve 10 and a second flow passage selector valve 11 each comprising a solenoid valve. The first and second flow passage selector valves 10, 11 are adapted to selectively change over the respective wheel cylinders 3, 4 to communication with the output port 2a of the first control valve 2 or with a reservoir tank 12. In the usual state shown in FIG. 5, the first and second pas sage selector valves 10, 11 are set to communicate the wheel cylinders 3, 4 with the output port 2a and to cut off the wheel cylinders 3, 4 from the reservoir tank 12.
The first and second supply passageways 8, 9 are further provided, at positions upstream of the first and second flow passage selector valves 10, 11, with respective first and second pressure-compensated flow control valves 13, 14, each comprising a mechanical valve, for selectively communicating and cutting off the output port 2a and the wheel cylinders 3, 4. The first and second pressure-compensated flow control valves 13, 14 are adapted to open the first and second supply passageways 8, 9 under the ordinary conditions shown in FIG. 5, and to change over and cut off the first and second supply passageways 8, 9 when a predetermined pressure differential develops between the pressure on the side of output port 2a and the pressures acting upon the wheel cylinders 3, 4.
The output port 2a is equipped with a first orifice 15 for communicating it with the wheel cylinder 3 via a third supply passageway 16 through the first flow passage selector valve 10, which bypasses tee first pressure-compensated flow control valve 13, and with a second orifice 17 for communicating it with the wheel cylinder 4 via a fourth supply passageway 18 through the second flow passage selector valve 11, which bypasses the second pressure-compensated flow control valve 14.
The second supply passageway 9 is provided with a proportioning valve PV 19 at a point between the second flow passage selector valve 11 and the wheel cylinder 4.
The reservoir tank 12 is connected to the suction side of the pump la, whose discharge side is in communication with the accumulator 1b and an input port 2b of the first control valve 2. The first and second control valves 2, 5 are connected to a brake pedal 21 via an equalizer 20. When the brake is inoperative, which is the state shown in FIG. 5, the first and second control valves 2, 5 are set at a first position to communicate the output port 2a with a discharge port 2c. By stepping down on the brake pedal 21, the valves 2, 5 are set at a second position where the three ports 2a, 2b, 2c are cut off from one another. By stepping down on the brake pedal 21 further, the valves 2, 5 are set at a third position where the output port 2a and input port 2b are communicated.
Depending upon the operating states of the first flow passage selector valve 10 and first pressure-compensated flow control valve 13, the antiskid brake control system is set to the control modes shown in Table 1.
TABLE 1 ______________________________________ ANTISKID BRAKE CONTROL MODES FLOW PASSAGE SELECTOR VALUE 10 ON OFF ______________________________________ PRESSURE- ON SLOW SLOW COMPENSATED DEPRESSUR- PRESSUR- FLOW CONTROL IZATION IZATION VALVE 13 OFF FAST FAST DEPRESSUR- PRESSUR- IZATION IZATION ______________________________________
The operation of this antiskid brake control system will now be described.
Ordinarily, the valves 10, 11, 13 and 14 are in the states shown in FIG. 5, and the output port 2a is in communication with the wheel cylinders 3, 4. Accordingly, when the brake pedal 21 is depressed to set the first control valve 2 at the third position at the time of braking, the hydraulic pressure of the hydraulic-pressure source 1 is introduced to the wheel cylinder 3 for the right front wheel and the wheel cylinder 4 for the left rear wheel via the first and second supply passageways 8, 9, thereby applying braking to the right front wheel and the left rear wheel.
When it is detected that the right front wheel is about to skid, the solenoid of the first pressure-compensated flow control valve 13 is energized in response to the resulting detection signal. The first pressure-compensated flow control valve 13 therefore shuts the first supply passageway 8. Consequently, the brake fluid is supplied to the wheel cylinder 3 for the right front wheel solely through the third supply passageway 16 having the orifice 15. Accordingly, the brake hydraulic pressure of the wheel cylinder 3 gradually rises. This is the slow pressurization mode. If the right front wheel is still about to skid even in the slow pressurization mode, the solenoid of the first passage selector valve 10 is energized to change over this selector valve, and the first pressure-compensated flow control valve 13 is changed over to the ordinary state. As a result, the output port 2a and the wheel cylinder 3 are cut off from each other, the passageway of the brake fluid is changed over and the first supply passageway 8 is opened so as to communicate the wheel cylinder 3 with the reservoir tank 12. Consequently, the brake fluid inside the wheel cylinder 3 rapidly flows out to the reservoir 12, as a result of which the brake hydraulic pressure of wheel cylinder 3 rapidly declines. This is the fast depressurization mode, in which the brake pressure acting upon the right front wheel decreases at a rapid rate. When the brake hydraulic pressure of the wheel cylinder 3 diminishes by a predetermined amount, the first pressure-compensated flow control valve 13 changes over again to shut the first supply passageway 8. Accordingly, the brake fluid inside the wheel cylinder 3 gradually flows out to the reservoir tank 12, as a result of which the brake hydraulic pressure of the wheel cylinder 3 gradually declines. This is the slow depressurization mode, in which the brake pressure acting upon the right front wheel diminishes in gradual fashion.
When the right front wheel is no longer in danger of skidding, the solenoids of both valves 10, 13 are turned off to communicate the output port 2a and wheel cylinder 3, and the passageway of the brake fluid is changed over so as to cut off the wheel cylinder 3 and reservoir tank 12 from each other, and the first supply passageway 8 is opened. As a result, the brake hydraulic pressure of the wheel cylinder 3 rises at a rapid rate to establish the fast pressurization mode, in which the braking force acting upon the right front wheel rapidly increases.
Thus, brake hydraulic pressure is controlled so as to eliminate skidding and make braking distance as short as possible. The control characteristic in this case is shown in FIG. 6.
Antiskid control is performed in a similar manner also when the other wheels are about to skid.
With this antiskid brake control system, however, control is carried out in the four modes shown in Table 1, and hence there is a transition from a pressurizing mode directly to a depressurizing mode or from a depressurizing mode directly to a pressurizing mode. Consequently, when there is a transition from one mode to another, there is a large fluctuation in G (the rate of the adjustment) as well as a large fluctuation in the braking hydraulic pressure. This results in a large amount of brake-pedal kickback as well as a deterioration in riding comfort and driving feeling.
In addition, if it is attempted to minimize the fluctuation in the braking hydraulic pressure, the flow passage selector valve 10 and the first pressure compensated flow control valve 13 must be finely controlled, and therefore the number of times these valves are actuated increases. As a consequence, the flow passage selector valve and first pressure-compensated flow control valve must be highly durable. This results in higher cost.